As the topic states, what tells the PCM what RPM your engine is operating at?

Today I was cruising along at exactly 45 mph and when lightly accelerating the RPM gauge wasn't moving even though I could feel the car pulling and increased my cars speed to 50 mph.

 

ther'es a white wire from the Ign. control module to the pcm, which sends a signal to the gauge. I would get it scanned to see if the rpm in the pcm matches the rpm on your gauge all the time like it should. if not, then there's something goin on in the circuit from the pcm to the gauge, or the cluster itself is bad.
chris

 

I should be able to do data logging by the end of the week (assuming the post office didnt "damage my mail" and suddenly the check is missing again), so I guess I'll have to wait until then.

 

you need to see real time data for comparasons sake.

 

Yeah sorry I should be able to do that with freescan and/or datamaster... or at least thats how it seems by looking through the programs.

 

If you have an auto (which it sounds like) that is quite normal with light pedal pressure. I can give my car 1/4 throttle and it will accelerate but stay at 2K for a bit.

 

Umm, I thought the tach was controlled by the low pulse resolution received from the opti?

 

Yes..... the RPM signal is derived in the PCM from the low res Opti pulse. It sends the "fire" signal to the ICM via the white wire.... that is not an input to the Opti.